The present invention relates to a method and a device for generating a vibrating signal.
In the mobile telephones and pagers of today, a silent alert signal is often desired. For example, in the theatre or in the restaurant, a ring signal can disturb other persons.
Therefore, some mobile telephones or pagers are equipped with a silent alert signal generator for generating a silent alert signal, such as a light signal or a vibration signal. In the case when the silent alert signal is generated as a vibrating signal, a vibrator must be provided.
A vibrator usually consists of a small electric motor and an eccentric loaded excitation mass. The vibration level of such a vibrator is not proportional to the speed of the motor. Instead, since the vibrator is placed in a telephone or a pager it will be suspended therein, and will thus form a mass-spring-system having a resonance frequency. Therefore, the maximum vibration level will be obtained when the motor speed coincides with the resonance frequency.
However, since the only known parameter in the resonance system is the mass of the pager or telephone, there is a problem of finding the resonant frequency in order to drive the motor to this frequency and thereby obtaining a maximum vibration level. This is particularly true when the mobile telephone or the pager is placed in a pocket or the like, which then forms a combined device-environment resonance system having many unknown parameters.
In order to drive the motor to a suitable speed, a control circuit can be used. A sensor is used to find the frequency corresponding to the maximum resonance level. The control system is then set to drive the motor to a speed corresponding to the resonance frequency. An advantage of using such a control system is that the system is able to compensate for a change in the resonance frequency. However, the use of a control system will make the vibrating device more expensive.
Another, more straight forward, solution to the problem of obtaining a high vibration level, is to increase the radius and hence the mass of the excitation lump. The use of a larger mass is however not desired in today""s mobile telephones and pagers, where requirements on low weight often exist.
Also, in U.S. Pat. No. 5,436,622 a vibrating signal device is disclosed. The object of U.S. Pat. No. 5,436,622 is to obtain vibration frequencies for the device and in particular the optimum vibration frequency using a feed-back circuit. In order to find the optimum frequency, a signal corresponding to a number of frequencies is fed to the device. Also, the device is capable of varying the frequency generated by the vibrating signal in order to make it possible to generate distinctive tactile sensations. The device can however not easily be made of very low weight and at the same time provide a vibrating signal that is strong enough for many applications, such as mobile telephones and pagers.
It is an object of the present invention to provide an improved vibrating device, which overcomes the problems outlined above, and which can provide a vibrating signal which is easy to sense and which at the same time makes it possible to keep the vibrating device very small and of low weight.
This object is obtained by a method of controlling a vibrator wherein the vibrator first is driven towards a first frequency during a first interval and then is driven towards a second lower frequency during a second interval. The first frequency is higher than the resonant frequency of the vibrating device, and the second frequency is lower than the resonant frequency of the device. A vibrating device is arranged to operate according to the method.
In a preferred embodiment the first frequency is higher than the lowest resonance frequency of the combined vibrating system formed by the vibrator suspended in a device, such as a mobile telephone or a pager and the environment where it is placed, such as a pocket or a belt holder or the like.
The vibrator is then allowed to reduce the frequency to a frequency lower than the resonance frequency of the combined system. Particularly the second frequency is significantly lower than the first frequency such as in the range 25%-75%, or even lower, for example less than 10% of the first frequency.
By letting the frequency vary in a pulsating manner in the vibrating device, it has surprisingly been shown that the vibration can be sensed more easily by a human, and thereby making it possible to use a very small mass in the vibrating device, which therefore can be made using a small weight, while still making it possible to sense the vibration.
In some applications, it may be advantageous to let the pulsating power signal applied to the motor of the vibrator only drive the motor to a speed corresponding to a maximum frequency, which is significantly lower than a resonance frequency of the vibrator or the combined vibrator-environment system.
Also, the method of controlling a vibrator is not limited to a particular type of vibrator, but can be used for controlling any existing vibrator in order to make the vibration easier to sense.